It's a boost of chemical substances in a concentration much higer than what you get from normal intake, and might affect you in several ways. Some of them beneficial, some not. Echinacea is a natural product, often advised in winter times as a natural way to strengthen the immune system. In any form I've tried it in, it gives me extreme stomach aches. There is one brand of bottled water which always gives me diarhoea, for no other reason than that it has certain elements in a much higher concentration than other brands of bottled water. Certain innocent herbs or nuts have the same effect on me as downing half a bottle of vodka or having a hangover. So while ZMA may not be a drug according to your definition, a significantly increased intake can have side effects. Btw, if ZMA is not a drug, you could argue that lithium is not a drug, yet lithium is used as an antidepressant. A normal body should get everything it needs from a balanced diet. Adding high isolated doses of specific things is going to have an effect (which is what you're hoping for or you wouldn't take them) but they're also going to have side effects. Just saying it's not a drug will not change that.